1. Field of the Invention
This invention relates in general to antenna systems, and in particular to a dual gridded reflector antenna system.
2. Description of Related Art
Communications satellites have become commonplace for use in many types of communications services, e.g., data transfer, voice communications, television spot beam coverage, and other data transfer applications. As such, satellites must provide signals to various geographic locations on the Earth's surface. As such, typical satellites use customized antenna designs to provide signal coverage for a particular country or geographic area.
In order to provide good cross-polarization performance over the geographic region of interest, a shaped dual reflector geometry is often used. The subreflector and/or main reflector is then shaped to generate a beam pattern that covers the intended coverage geographic region.
An advantage of dual reflector designs is that the main reflector is thin and therefore generally easy to package and stow in the confines of the launch vehicle volume constraints. A typical dual reflector antenna system can provide one beam for each of two linear polarizations. However, typical dual reflector antenna systems have a main reflector that has only one solid surface, and therefore can generate only one distinct beam shape.
Alternately, a "dual-gridded" shaped reflector system may be used to produce beams over the desired coverage area. This type of antenna system is a shared aperture system having two separate reflective surfaces, one reflective surface for each polarization. Each reflective surface, also called a "front shell" and a "rear shell," may be shaped to produce a distinct beam shape for each polarization. The cross-polarization performance is a function of both the front and rear shell geometry. To provide adequate cross-polarization performance, the two focal points must be separated. The resulting reflector shell becomes large and thick, and therefore difficult to package and stow within the confines of the launch vehicle constraints. The use of multiple antennas can also produce multiple beam patterns, however, multiple antennas within a system also produce space and deployment problems for the satellite and make it difficult to design the satellite to fit within the launch vehicle volume constraints.
It can be seen, then, that there is a need in the art for antenna reflectors that provide multiple distinctly shaped beams. It can also be seen that there is a need in the art for antenna systems that provide distinctly shaped beams for multiple polarizations that are easy to stow within launch vehicle constraints.